In a typical compressor for a cryogenic refrigerator helium returns from a cryogenic refrigerator to a compressor pump via a helium return line. Oil is injected into the helium at the inlet to the compressor. The oil absorbs the heat of compression given off by the helium. The combined mixture of helium and oil is pumped from the compressor through a line to a heat exchanger where the heat contained in the mixture is given off. The helium and oil mixture is then pumped to a bulk oil separator which separates the helium from the oil and the oil returns via a line back to the compressor. The helium travels from separator to an oil mist separator where any residual oil mist is separated from the helium.
The helium travels from the oil mist separator to an adsorber which further removes any remaining impurities from the helium. From the adsorber, the helium is then pumped via a helium supply line to the cold head of a cryogenic refrigerator such as a Gifford-McMahon cryogenic refrigerator. The helium travels through the cryogenic refrigerator and returns via the helium return line back to the compressor where the cycle is again repeated.
An additional helium line lies between the helium supply line and the helium return line. Situated within this line is a differential-pressure relief valve. The line and valve are located in between the helium return line and the helium supply line. Any excess pressure which may build up in the helium supply line to the cryogenic refrigerator can be released through this line and valve and shunted to the helium return line valve. The relief valve automatically opens and allows helium to travel from the supply line to the return line when the pressure fluid within the helium supply line reaches a given predetermined pre-set pressure.
However, the present in-line differential-pressure relief valves must be pre-set on a test board and built into the line of a compressor since all adjustments are internal. When the compressor is placed into operation and the settings within the relief valve are not correct or not matched to the compressor, the valve must be taken off the compressor and re-set on the test board. This practice is costly and wasteful of gas and manpower. In addition, it is virtually impossible to optimize the performance of each compressor unit.